Jumping into the lecture circuit is a subject that makes Castro uncomfortable; he wouldn't say on the record how much money each speech will net him. A useful benchmark, though, is the career of Henry Cisneros, who also found it necessary as mayor of San Antonio in the 1980s to exchange his political capital for an actual livelihood.

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“I had to make a living,” Cisneros said of the $10,000 he earned per speech while mayor. “And the mayor's job in and of itself makes it difficult to take a full-time job because you're either committed full-time or you're in conflict.

“I had people offer me positions, but it would have been in conflict with my city duties.”

Castro knows this quandary firsthand.

Start with his salary.

Last year, he earned about $4,000 as mayor — not quite enough for a married father. (Castro's wife, Erica, makes $55,000 a year as an elementary school teacher.)

Castro, 38, started earning a pittance more than a decade ago as a council member.

A graduate of Harvard Law School, he once worked as an attorney with the San Antonio office of Akin Gump Strauss Hauer & Feld, a job that paid him $98,000 a year.

But Castro resigned less than a year after his 2001 election to the City Council. It had become clear that his job at the powerful law firm would prevent him from voting on too many issues, notably a proposed PGA Village development over the Edwards Aquifer recharge zone.

(The developer had hired Akin Gump to draft the development agreement for City Council approval. After he resigned, Castro cast the lone dissenting vote on council.)

“Public service was always my compromise,” Castro said. “I felt like I couldn't be happy just making my money. I wanted to be doing something in the service of other people, too.”

The consequence of this desire for public service was nearly catastrophic: “My house almost went into foreclosure,” he says.

After leaving the City Council in 2005, Castro started a private practice as a trial attorney. He earned a much-ballyhooed seven-figure referral fee in 2007. But he started phasing out his practice once elected mayor in 2009.

That meant returning gradually to the pittance the city pays its elected officials: $20 a meeting, capped at $1,040 a year. (The mayor earns an additional $3,000 a year.)

Enter Castro's star turn as keynote speaker last summer at the Democratic National Convention, where his position at City Hall was transfigured into a national brand. Now Castro has an agent (William Morris Agency), a book deal and the potential to make a comfortable living merely on the lecture circuit.

How comfortable?

Cisneros says the $10,000 a speech he earned in the 1980s “was enough to offset the fact that I couldn't take another job.”

“So I was able to do one speech a month,” he said, “and cover my family responsibilities and be very careful with my time away from town.”

Castro confirmed his fee is in the same financial ballpark as Cisneros'. So far, he's accepted three gigs: one with the NDC, another with the American Association of Hispanic Advertisers and a third at the University of Arkansas.

At most, Castro will speak for money once a month, he says, and never in San Antonio.

This month, the New York Times reported that Newark Mayor Cory A. Booker has earned about $1 million from public speaking engagements during his seven years in office.

Critics have accused Booker — like Castro, a rising Democratic star — of neglecting his official duties for his national ambitions.

There's a big difference between these two mayors, though: As eminence of a city of about 280,000 residents, Booker earns a salary of about $135,000 a year.

Like Booker, Castro might have national ambitions. But like Cisneros, he also has a family to support.